It should be understood that the term "light modulating devices" is used in this specification to encompass both light transmissive light modulators, such as diffractive spatial modulators or conventional liquid crystal displays, light emissive modulators, such as electroluminescent or plasma displays, reflective or transflective devices or displays, and other spatial light modulators, such as optically addressed spatial light modulators or plasma addressed spatial light modulators.
Liquid crystal devices are commonly used for displaying alphanumeric information and/or graphic images. Furthermore liquid crystal devices are also used as optical shutters, for example in printers. Such liquid crystal devices comprise a matrix of individually addressable modulating elements which can be designed to produce not only black and white transmission levels, but also intermediate or "grey" transmission levels. In color devices, such as those employing color filters, such intermediate or grey transmission levels may be used to give a wider variety of colors or hues. The so-called grey scale response of such a device may be produced in a number of ways.
For example the grey scale response may be produced by modulating the transmission of each element between "on" and "off" states in dependence on the applied drive signal so as to provide different levels of analogue grey. In a twisted nematic device, for example, the transmission of each element may be determined by an applied RMS voltage and different levels of grey may be produced by suitable control of the voltage. In active matrix devices the voltage stored at the picture element similarly controls the grey level. On the other hand, it is more difficult to control the transmission in an analogue fashion in a bi-stable liquid crystal device, such as a ferroelectric liquid crystal device, although various methods have been proposed by which transmission may be controlled by modulating the voltage signal in such a device. In devices having no analogue grey scale, a grey scale response may be produced by so-called spatial or temporal dither techniques, or such techniques may be used to augment the analogue grey scale.
In a spatial dither (SD) technique each element is divided into two or more separately addressable sub-elements which are addressable by different combinations of switching signals in order to produce different overall levels of grey. For example, in the simple case of an element comprising with two equal sized sub-elements each of which is switchable between a white and a black state, three grey levels (including white and black) will be obtainable corresponding to both sub-elements being switched to the white state, both sub-elements being switched to the black state, and one sub-element being in the white state while the other sub-element is in the black state. Since both sub-elements are of the same size, the same grey level will be obtained regardless of which of the sub-elements is in the white state and which is in the black state, so that the switching circuit must be designed to take account of this level of redundancy. It is also possible for the sub-elements to be of different sizes so as to constitute two or more spatial bits of different significance, which will have the effect that different grey levels will be produced depending on which of the two sub-elements is in the white state and which is in the black state.
In a temporal dither (TD) technique at least part of each element is addressable by different time modulating signals in order to produce different overall levels of grey within the addressing frame. For example, in a simple case in which an element is addressable within the frame by two sub-frames of equal duration, the element may be arranged to be in the white state when it is addressed so as to be "on" in both sub-frames, and the element may be arranged to be in the dark state when it is addressed so as to be "off" in both sub-frames. Furthermore the element may be in an intermediate grey state when it is addressed so as to be "on" in one sub-frame and "off" in the other sub-frame. Alternatively the sub-frames may be of different durations so as to constitute two or more temporal bits of different significance. Furthermore it is possible to combine such a temporal dither technique with spatial dither by addressing one or more of the sub-elements in a spatial dither arrangement by different time modulated signals.
Temporal dither relies on the observer's eye averaging a series of periods of different transmission levels, for example black and white transmission levels, so as to perceive a particular grey level. As long as the transitions from one transmission level to any other are faster than the eye integration period no flicker is observed. However, if the transition period is close to the eye integration period, problems may be experienced when an element changes from one temporal grey level to another temporal grey level due to the tendency of the eye to integrate through the transition period between the two grey levels and the fact that a particular sequence of temporal bits over the transition may add up to a perceived transmission level which is not between the grey levels on either side of the transition, that is which is either greater than or less than both of these grey levels. Although such a transitional transmission level is not generally perceived in complex video images, this phenomenon may give rise to an artifact which becomes observable when smooth gradients in grey level move across a display. Since many transitions between grey levels may give rise to such an incorrect transitional grey level, and the amplitude of the error is largest in the case of transitions between consecutive grey levels, a bright or dark false contour, which may be termed a pseudo-edge, may be perceived in a moving image as a result of such a phenomenon. FIG. 5 diagrammatically illustrates the running average transmission level in such a case during a transition from a starting grey level 1 to an adjacent finishing grey level 2. In this case the transitional grey level passes first through a region 3 in which it is higher than both of the grey levels 1 and 2 and then through a region 4 in which it is lower than both of the grey levels 1 and 2, thus resulting in incorrect grey levels being observable momentarily during such a transition.
Y-W. Zhu et al., "A Motion-Dependent Equalizing-Pulse Technique for Reducing Dynamic False Contours on PDPs" Technical Report of IEICE. EID 96-60 (1996-11), pp. 67-72 discloses a means for compensating for dynamic false contours by an equalizing-pulse technique in which light emission periods are added or subtracted during such a transition in order to compensate for an erroneous transmission level less than or greater than the intended transmission level. However such a technique requires the use of a complicated algorithm which must include the speed of motion in the video signal in order to provide effective compensation.